Magnetic recording media are widely used as audio tapes, video tapes, floppy disks, etc. A magnetic recording medium is fundamentally composed of a non-magnetic support having formed thereon a magnetic layer comprising a powdery ferromagnetic material dispersed in a binder.
Hitherto, a magnetic recording medium such as a magnetic recording tape, etc., is generally produced by coating a coating composition comprising a dispersion of a ferromagnetic powder dispersed in a binder dissolved in a solvent on a web-form non-magnetic support continuously travelling in the lengthwise direction and, after drying the coated layer, cutting the support having the magnetic layer formed thereon.
However, in the production of a magnetic recording tape, to increase the sensitivity and improve the S/N ratio thereof, it is necessary to make the directions of ferromagnetic powders uniform in the travelling direction of the non-magnetic support to increase the squareness ratio (the value obtained by dividing the residual magnetization Br by the saturation magnetization Bm) of the coated magnetic layer. Hence in producing a magnetic recording tape, etc., a process has hitherto been employed wherein a magnetic field is applied to the coated layer while the layer is in wet condition in the travelling direction of the nonmagnetic support by a permanent magnet or a solenoid to render uniform the direction of the easy magnetization axes of the ferromagnetic powders (i.e., to orient the ferromagnetic powders).
A magnetic recording medium had to have high levels of various characteristics, such as electromagnetic conversion characteristics, running durability, running performance, etc., thereof. More specifically, in audio tapes for recording and reproducing music, a higher original-reproducing capability has been required and also in video tapes, excellent electromagnetic conversion characteristics such as excellent original image-reproducing capability, etc., have been required.
Also, recently, it has been strongly desired to increase the memory capacity and the recording and reproducing output of magnetic recording disks and magnetic recording tapes.
To increase the memory capacity, it is, as a matter of course, necessary to increase the information recording density per unit area of a magnetic recording medium. On the other hand, to increase the information recording density, the writing magnetic flux generated from a magnetic head must be concentrated in a fine area. Thus, the magnetic head must be minimized and the amount of the magnetic flux generated therefrom reduced.
For inverting the direction of magnetization of a magnetic layer by the fine amount of the magnetic flux thus reduced, it is necessary to reduce the volume of the magnetic layer. Hence it can be said that if the thickness of the magnetic layer is not reduced, complete magnetization inversion cannot be formed.
By the aforesaid reason, for meeting the aforesaid requirement, the thickness of the magnetic layer must be reduced.
On the other hand, to increase the recording and reproducing output of a magnetic layer, the residual magnetic flux of the magnetic layer must be reduced. One method to achieve this is to increase the thickness of the magnetic layer, however, if the thickness of the magnetic layer is increased, high frequency characteristics are deteriorated. Accordingly, to increase the residual magnetic flux and to improve high frequency characteristics, the coercive force of the magnetic material must be increased in addition a reduction in the thickness of the magnetic layer. That is, when the thickness of a magnetic layer containing a magnetic material having a high coercive force is reduced, the high frequency characteristics are improved but the low frequency characteristics are reduced. Furthermore, if the thickness of a magnetic layer containing a magnetic substance having a high coersive force is increased, a magnetic field acts on the surface thereof only at high frequency, whereby a record recorded on the magnetic layer tender to be difficult to erase.
Thus, as a magnetic recording medium having both characteristics, a magnetic recording medium composed of a support having formed thereon a relatively thick magnetic layer containing a magnetic material having a relatively low coercive force and a large residual magnetic flux and a thin magnetic layer containing a magnetic material having a high coercive force formed on the thick magnetic layer is ideal. Thus, hitherto, production of a magnetic recording medium by coating two magnetic layers and applying thereto a magnetic orientation treatment has been employed. However, in coating two magnetic layers, it has been confirmed that when the dry thickness of the upper magnetic layer is thinner than 2 .mu.m, the characteristics, such as the squareness ratio, is not so improved as compared in forming an upper magnetic layer having a dry thickness thicker than 2 .mu.m.
Thus, to solve these problems, JP-A-62-42328 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") provides a process capable of providing superior electromagnetic conversion characteristics by double-coating a first magnetic layer and a second magnetic layer on a support in this order and in this case, a magnetic material having a high coercive force (Hc) is present in the second magnetic layer and a magnetic material having a low Hc is present in the first magnetic layer.
Also, JP-A-62-212933 describes that where two or more magnetic layers are formed on a non-magnetic support, the dry thickness of the uppermost layer being not thicker than 2 .mu.m, a magnetic recording medium having high characteristics is obtained by simultaneously coating two or more magnetic layers using a simultaneous multi-coating method (wet-on-wet method) and applying thereto a magnetic field while the layers are in a wet condition. Hitherto, it is known that the binder plays an important role in the dispersion of a ferromagnetic powder and a vinyl chloride series resin or a cellulose series resin is usually used as the binder. Thus, in JP-A-62-212933 above, a vinyl chloride series resin or nitrocellulose is used in combination with polyurethane which is a relatively soft binder. However, the combination of these binders results in a somewhat insufficient surface roughness of the magnetic layer as well as Y.S/N (video S/N) and C.S/N (color S/N) thereof and a further improvement has been required.
To solve these problems, it has been proposed to use the binder present in the upper layer (second magnetic layer) and the binder present in the lower layer (first magnetic layer) in a same ratio to the amount of the ferromagnetic powder present in each layer as disclosed in JP-A-62-212933 and JP-A-54-145104 or to use the binder for the upper layer in an amount larger than the amount of the binder for the lower layer (about 25% larger) (the amount of the binder being the amount per unit amount of the ferromagnetic powder) as disclosed in JP-A-58-56231. However, when the binders are used in a same ratio between the upper layer and the lower layer, the surface roughness may be good but Y.S/N and C.S/N are insufficient. Also when the difference in the amount of the binder between the upper layer and the lower layer is large, it has been confirmed that not only surface roughness is increased but also Y.S/N and C.S/N are greatly reduced.
Also, at present, a magnetic recording medium must have a good running durability and the durability of the magnetic recording medium itself, that is, an improved durability with the passage of time under various temperature and humidity conditions, in addition to excellent electromagnetic conversion characteristics.
To obtain good running durability, the functions of abrasives and lubricants usually play an important role.
However, to obtain excellent running durability in a magnetic recording medium using abrasives, it is necessary to increase the addition amount thereof to some extent. This results in a decrease in the content of a ferromagnetic material. Also, in using an abrasive having a large particle size for obtaining excellent running durability, the abrasive tends to protrude excessively beyond the surface of the magnetic layer. Accordingly, improvement in running durability using abrasives is frequently accompanied by a reduction in the electromagnetic conversion characteristics.
Also, in improving the aforesaid running durability using a lubricant, it is also necessary to increase the amount thereof. Thus the binder tends to be plasticized and there is a tendency toward a reduction in the durability of the magnetic layer.
Another method for improving the running durability includes a method of increasing the hardness of the magnetic layer by using a hard binder. However, an increase in the hardness of the magnetic layer adversely influences the magnetic recording medium because the magnetic layer becomes quite brittle, dropout occurs on contact of the magnetic layer with a magnetic head, and the still characteristics of the magnetic recording medium are reduced.
Furthermore, a magnetic recording medium having a binder composition composed of polyisocyanate, a polyurethane series resin having a hydroxy group and a molecular weight of less than 10,000, and a resin having an active hydrogen is disclosed in JP-A-58-153224. A polyurethane series resin (or other resins) having an active hydrogen (such as a hydroxy group) sufficiently reacts (crosslinkage) with polyisocyanate due to the high reactivity with polyisocyanate. As a result, the magnetic layer containing the resin becomes very tough. Accordingly, a magnetic layer having less reduction in output and good durability can be obtained even after storing the magnetic recording medium for a long period of time. However, since this magnetic layer has insufficient adhesive power for a non-magnetic support, a sufficient running durability is not always obtained.
As a magnetic recording medium having both excellent electromagnetic conversion characteristics and running durability, JP-A-63-103429 discloses a magnetic recording medium having on a non-magnetic support two magnetic layers. Each contains a ferromagnetic substance having a coercive force of at least 500 Oe dispersed in a binder, wherein the Young's modulus of the lower magnetic layer (first magnetic layer) formed on the non-magnetic support is from 500 to 1,000 kg/mm.sup.2, the Young's modulus of the upper magnetic layer (second magnetic layer) formed on the lower magnetic layer is at least 1,300 kg/mm.sup.2, and the Young's modulus of the all of the magnetic layers is at least 900 kg/mm.sup.2. In this magnetic recording medium, the lower magnetic layer has a high buffer action and a good adhesive property due to the softness thereof. Also the upper magnetic layer has a high hardness, thereby the magnetic recording medium is difficult to deform even when it is stored under a high-temperature condition and the running durability thereof is improved.
To obtain such a Young's moduluses in these magnetic layers, a method wherein substantially the same amount of a ferromagnetic material is used for the first magnetic layer (lower layer) and the second magnetic layer (upper layer), and a binder having a Young's modulus lower than the binder for the second magnetic layer is used for the first magnetic layer, a method wherein a same binder is used for the first magnetic layer and the second magnetic layer and the amount of a ferromagnetic material for the first magnetic layer is lower than the amount of a ferromagnetic substance for the second magnetic layer, or a method wherein each of the Young's modulus of the first magnetic layer and the Young's modulus of the second magnetic layer is adjusted by controlling the amount of ferromagnetic material and the kind of the binders for both the magnetic layers can be utilized.
However, JP-A-63-103429 above practically shows that a desired Young's modulus is obtained by using the same binder for the first magnetic layer (lower layer) and the second magnetic layer (upper layer) and changing the amounts of a vinylchloride/vinyl acetate/vinyl alcohol copolymer, which is a relatively hard binder, and polyester urethane, which is a relatively soft binder. Thus with such a combination of polymers, since in the lower layer (first magnetic layer) having a small Young's modulus, the amount of the vinyl chloride copolymer having a relatively good dispersibility becomes less and the amount of polyurethane having a poor dispersibility becomes relatively large, the dispersibility of the lower layer tends to be reduced. Due to the influence thereof, a sufficient surface property of the upper layer (second magnetic layer) is not obtained and, as the case may be, the strength of the upper layer is insufficient.
Furthermore, in an attempt to improve the durability of a magnetic recording medium by improving the chemical structure of a binder to introduce therein a bond which is reluctant to be hydrolyzed, a magnetic recording medium using, for example, a polyurethane resin containing a polyether diol in the molecule is disclosed in JP-A-61-255532. The magnetic recording medium containing such a polyurethane resin has excellent electromagnetic conversion characteristics and shows an excellent durability to the change of temperature and humidity owing to its resistivity to hydrolysis. However, the coefficient of friction is increased and the output is reduced on running. Thus, this magnetic recording medium cannot be said to have sufficient running durability.
As described above, a magnetic recording medium having excellent electromagnetic conversion characteristics, an excellent running durability such that the reduction of the output on running is less, etc., and an excellent durability such that the reduction of output is less even when the magnetic recording medium is stored under various temperature and humidity conditions for a long period of time has not yet been obtained.
On the other hand, to increase the durability, there is a method of using a hardening agent such as an isocyanate, etc. In producing a magnetic recording tape having a double layer structure, a coating after drying system has hitherto been employed. That is, in this coating system, a first magnetic layer is coated on a non-magnetic support and after drying the coated layer, a second magnetic layer is coated and dried. It is known, however, that in this system, by using a large amount of hardening agent for the first magnetic layer, the occurrence of a nonuniform swelling at the interface even when the second magnetic layer is formed after setting of the first magnetic layer. For example, it is disclosed in JP-A-57-135434 that a hardening agent (isocyanate) is used in an amount of 3.75 parts by weight for the last magnetic layer and 1.25 parts by weight for the second magnetic layer based on 100 parts by weight of ferromagnetic substance.
However, recently, a wet-on-wet coating (simultaneous double coating) system has been developed for reducing the thickness of the second magnetic layer, improving the surface property of the first magnetic layer and the second magnetic layer, improving the coating speed, and reducing the production cost.
In this system, it becomes unnecessary to use a large amount of a hardening agent for the first magnetic layer for hardening the layer since the first magnetic layer is formed on the second magnetic layer while the second magnetic layer is in a wet condition and, for example, JP-A-53-144705 discloses that a larger amount of a hardening agent is used for the second magnetic layer.
However, it has recently been found that when the magnetic recording medium is used or stored under various temperature and humidity conditions, the polyurethane resin in the medium is hydrolyzed to form low-molecular weight components, which ooze to cause blooming, etc. When a large amount of a hardening agent is used for the second magnetic layer for preventing the occurrence of this phenomenon, the magnetic layer becomes very hard and hence a sufficient head touch, i.e., the touch of a magnetic head and the magnetic recording tape, is not obtained. Also, good electromagnetic conversion characteristics are not obtained as a result.